Fixed bed coal gasification

ABSTRACT

THE GASIFICATION OF A CAKING COAL IN A FIXED BED IS EFFECTED BY FEEDING A SUITABLE MIXTURE OF COARSE CAKING COAL AND NON-CAKING PELLETS OF SUBSTANTIALLY THE SAME SIZE MADE BY PELLETIZING FINES, PREVIOUSLY SEPARATED FROM THE CAKING COAL, IN A HOT PELETIZING ROTARY KILN, WHEREBY SUBSTANTALLY IMPROVED FEEDSTOCK IS PROVIDED FOR THE FIXED BED GASIFIER.

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INVENTOR.

E. H. REICHL United States Patent 3,692,505 FIXED BED COAL GASIFICATION Eric H. Reichl, Pittsburgh, Pa., assignor to Consolidation Coal Company, Pittsburgh, Pa. Filed Apr. 5, 1971, Ser. No. 131,330 Int. Cl. C103 3/16 US. Cl. 48206 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the conversion of a caking coal to fuel gas, either high B.t.u. or relatively low B.t.u., by gasification of the caking coal in a fixed bed system.

Fixed bed systems are well known to be suitable for the conversion of coal to fuel gas. Such systems require a non-caking or only mildly caking feedstock for pratical operation. They generally use, as feedstocks, either noneaking or weakly caking coals, or the non-caking solid carbonaceous products resulting, for example, from low temperature carbonization of caking coals. Essentially the caking property of the coal reduces the capacity of the gasifier. This can be partly overcome by mechanical stirring devices. The overall decision thus becomes a compromise between caking tendency of feed mix, mechanical means to overcome caking, and gasifier capacity.

Because of the large reserves of caking bituminous coals in this country, it would be desirable to use such coals, many of which are very strongly caking, as feedstock to fixed bed gasifiers, with only minor alteration or revision of the gasifier itself.

In accordance with the present invention, caking coal, as received from a mine and having a spectrum of sizes but containing at least 5.0 percent by weight of fines is separated into two parts, coarse and fines. By fines, I mean coal having a size less than /s inch. The fines are fed to a substantially horizontal cylindrical rotary kiln wherein they are mixed with sufiicient hot, recycled non-caking carbonaceous solids (char) to raise the temperature adiabatically in a non-oxidizing atmosphere to a temperature within the range 700 to 900 F. The hot mixture is tumbled in the rotary kiln while progressing therethrough, whereby pellets of controlled size consist are formed. These pellets, without further heating, i.e. calcination, are non-caking and, surprisingly, are sufiiciently strong, without calcination, to withstand the pressures and abrasions of the fixed bed gasifier. The pellets are mixed with the coarse coal in suitable proportions to ensure an operable feed mix. The amount of pellets ranges widely from percent by weight of the total mix to 60 percent, depending on the caking properties of the coal. Where the amount of fines naturally occurring in the feed is inadequate, it will be necessary to crush some of the oversize to achieve the required coalpellet blend. The resulting mix then fed to the fixed bed of a gasifier of which there are many versions, the particular version not forming of itself part of this invention.

For a better understanding of my invention, reference should be had to the following description and the accompanying drawing which illustrates schematically the improved process of my invention.

Referring to the drawing wherein a preferred embodiment is schematically shown, numeral 10 designates a hot pelletizer, and numeral 12 designates a fixed bed gasifier.

Patented Sept. 19, 1972 The hot pelletizer is a substantially horizontal rotary kiln in which pellets may be formed by tumbling a mixture of caking coal and recycled carbonaceous solids known as char. The operation and structure of such a kiln are described in detail in the following US. patents:

The caking coal used as a feedstock in the process of the present invention is one which is generally not considered suitable as a feedstock to a fixed bed gasifier, both because of its caking properties and its content of fines. Its tendency to cake renders the fixed bed inoperaitve; and its fines content is blown out of the gasifier by the gas throughput. The coal feedstock contains at least 5.0 percent by weight of fines, and in general, the coal contains at least 35 percent by weight of fines having a top size of about inch, the remainder, the coarse fraction, having a top size of about 1 /2 inches.

The coal fedestock is conducted to a suitable screen separator 14 Where it is separated into a coarse fraction and a fines fraction. The latter fraction should contain at least that portion of the coal having a size less than A inch. It may contain some coal above that in size; the amount, if any, being a function of the optimum size selected for feed to the gasifier. The coarse fraction is conducted from the separator 14 by suitable means, a moving belt 16, for example, directly to the feed hopper 34 for the gasifier 12.

The fines fraction from the separator '14 is conducted by suitable means, a screw feeder 18, for instance, directly to the hot pelletizer 10. The fines may be introduced into the pelletizer without preheating, but if desired, some preheating may be effected, but only to a temperature below the softening point of the coal.

The temperature within the hot pelletizer is adiabatically maintained between 700 and 900 F. by means of a recycling stream of non-caking carbonaceous solids. These solids are the off-size product from the pelletizer, that is, those pellets which are either larger or smaller than those desired for transfer to the gasifier. If desired, the larger off-size may be comminuted by a crusher (not shown). These off-size pellets are recycled through a conduit 20 to a heater 22 which is adapted to confine a partial combustion zone wherein combustion is effected by means of air introduced through a conduit 24. The flue gas is exhausted through a pipe 26. The combustion in the heater is preferably conducted by circulating the air, as a fluidizing gas through a bed of the carbonaceous solids, and regulated to effect only partial combustion to raise the temperature of the solids sufficiently high to provide the adiabatic heat required in the pelletizer. Generally, if the ratio of hot solids to fines fed to the pelletizer is in the range of 2 to l to 3 to 1, the requisite temperature of the hot solids is in the range of 925 to 1400 F.

The hot solids are transferred from the heater 22 to the pelletizer 10 by a conduit 28 which generally includes a screw feeder (not shown) at its terminus within the pelletizer. By virtue of the rotation of the pelletizer, the hot solids and fines are tumbled together and form agglomerates, called pellets. These pellets are withdrawn from the exit end of the pelletizer into a suitable separator 30 which serves to recover a stream of pellets of the desired size consist or spectrum, i.e. A; by 2 inches, generally. These pellets are non-caking and possess a surprising strength despite the fact that they have not been calcined, that is, heated to temperatures in the neighborhood of 1600 F. They are conducted through a pipe 32 to a feed hopper 34 wherein they are conveniently mixed with the coarse coal from separator 14 in the relative proportions required to assure a non-calcing feed to the gasifier. These proportions for a strongly caking coal are in the range of about 20 to 60 percent by weight of pellets to about 80 to 40 percent by weight of coarse coal. If the caking coal is not strongly caking, the amount of pellets may be reduced to percent.

The volatile matter distilled from the coal in the pelletizer is Withdrawn through a conduit 36 to a condenser 38; condensed therein, except for some non-condensable gases; and conducted through a conduit 40 to a fractionator 42. The latter serves primarily to separate pitch irom distillate. The pitch may be used in the pelletizer, if needed to assist in the pelletization of the fines.

The fixed bed gasifier 12 receives the mixture of pellets and coarse coal from the feed hopper 34 through a conduit 44. The gasifier itself is a part of this invention to the extent that it is of the fixed bed type and requires a non-caking or mildly caking solid fuel for satisfactory commercial operation. The gasification agent consists of oxygen or air, and steam. It is introduced into the fixed bed of fuel through a conduit 46 at the bottom of the gasifier. Generally, the gasifier has a rotating grate at the bottom. The solid fuel is charged at the top continuously. It moves downward, as a fixed bed, slowly in countercurrent flow relationship to the steam and air/ oxygen. The rotating grate serves to remove ash. Fuel gas is withdrawn through conduit 48.

According to the provisions of the patent statutes, I have explained the principle, preferred construction, and mode of operation of my invention and have illustrated and described What I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. In a process for the gasification of a caking coal in a system using a fixed bed gasifier that requires a noncaking or mildly caking feedstock, the improvement which comprises separating the caking coal into a coarse fraction and a fines fraction, eifecting pelletization of said fines fraction in a substantially horizontal cylindrical rotating kiln wherein the fines fraction is heated adiabatically in a non-oxidizing atmosphere to a temperature within the range 700 to 900 F. by means of preheated non-caking carbonaceous solids, mixing pellets withdrawn from said kiln with the coarse coal fraction to form a non-caking or mildly caking feedstock, and introducing said feedstock into a fixed bed gasifier.

2. A process according to claim 1 wherein the caking coal contains at least 5 percent by weight of less than /3 inch coal; and the amount of pellets in said feedstock to said fixed bed gasifier ranges from 10 to percent by weight.

3. A process according to claim 1 wherein said preheated non-caking carbonaceous solids consist essentially of off-size recycled solids.

References Cited UNITED STATES PATENTS 749,302 1/1904 Koneman 48204 3,073,751 1/1963 Gorin et al. 2015 3,463,623 8/1969 Forney et al. 48-202 MORRIS O. WOLK, Primary Examiner D. G. MILLMAN, Assistant Examiner US. Cl. X.R. 

